An integrated silicon double bridge anemometer

Since 1974, silicon has been used for making flow sensors, though a lot of problems with respect to drift in time and temperature behaviour had to be solved. This is especially the case when the chip is operating at elevated temperatures. Heat conduction plays a vital role in the general heat transfer characteristics of the flow sensor. Due to a special mounting technique, this conduction has been reduced to less than 6% of the total power dissipation. To improve drift and temperature behavior, a fundamental attempt has been made to split up the common mode (fault) signal and the differential flow signal. This goal is achieved by integrating two equal Wheatstone bridges on one silicon chip and connecting both bridges in opposite ways with respect to each other. This results in two flow signals with opposite polarity. Compared with a single bridge, reduced offset, a higher signal gain and better stability are achieved. The influence of ambient temperature can be compensated completely. A discussion of general heat transfer characteristics is presented together with results for static and dynamic behavior of the flow sensor.